1. Field of the Invention
The present invention relates to oil well optical apparatus. The present invention more particularly relates to fiber optical probes which are suitable for investigating fluid flow in a well.
2. State of the Art
The use of optical systems for the analysis of fluids is well known. For example, as set forth in the patents incorporated by reference above, optical probes can be used downhole for measuring oil, water, and gas holdup in three-phase flows. In particular, light of excitation is coupled to a small optical probe that is deployed into a sample flow. Depending on the optical properties of the fluid surrounding the probe, the returning signal carries the optical signature of the fluid. Gas will induce a large reflectance, compared with liquids, due to the large mismatch of the index of refraction. Crude oils, on the other hand, will produce fluorescence under illumination. By analyzing both the reflectance and the fluorescence signals, the nature of the fluid in contact with the probe can be identified.
As noted in the previously incorporated patents, using a flat tip probe (normal incidence), the contrast between the indices of refraction for water and oils is much more subtle than the contrast between the indices of refraction for gas and liquid. To enhance the contrast, off-normal incidence was proposed such as the 45° and 60° oblique tip designs and the 5°/50° biconical tip design. While the previously incorporated patents represent a major step forward in downhole analysis of fluids, the probes described therein are not necessarily optimal in certain circumstances.